It is well known to use acoustic sensors for monitoring pipelines e.g. to observe a wire breakage or similar. An example of such monitoring system is for example described in U.S. Pat. No. 6,082,193. This monitoring system comprises an array of acoustic sensors spaced along a cable and deployed in a fluid filled concrete pipeline. The sensors are monitored to find acoustic anomalies, particularly anomalies resulting from breakage of a reinforcing wire for the concrete. The location of wire breaks can be found from the data collected.
Acoustic monitoring systems have also been applied offshore. U.S. Pat. No. 7,751,977 describes a system for avoiding collision between a vessel and a manmade structure, where an acoustic sensor is connected or placed near to the manmade structure. The data measured by the acoustic sensor is transmitted wireless to the vessel.
WO 03/100453 describes an acoustic monitoring system with a number of hydrophones. By help of acoustic measurements the system can discover imbalances, vibrations and leakage. US 2009/0132183 describes a technique for monitoring a pipeline operatively connected to an optical fiber. The optical fibre may e.g. combine the observance of Brillouin backscatter and coherent Rayleigh noise.
EP 2006 654 discloses several methods for acoustic sensor leak detection of transmission and distribution pipes using hydrophones.
In many situations the prior art acoustic sensor systems work well. However, in general there is a need for an improved monitoring system for monitoring the integrity of a stationary structure and in particularly for integrity monitoring of stationary structures which should remain in position for a long time, such as several years.